From a coastal plain to an anthropized fluvial valley (NW Brittany, France): 7.3 kyr of paleoenvironmental evolution from sedimentological, palynological and paleogenomic perspectives
Résumé
Over the Holocene (last 11.7 kyr), the Brittany coastal region has undergone major changes in response to rising
sea level, climatic fluctuations and increasing anthropogenic influence. This study aims to i) accurately recon-
struct the paleoenvironmental evolution of a site located on the north Brittany coast (NW France), a sector that
has not been previously investigated thoroughly, and ii) improve the detection of the anthropogenic signature in
paleoenvironmental records by using a multidisciplinary approach. To achieve these goals, we made the first
cross-correlated study in coastal Brittany region, integrating sedimentological (grain-size, X-ray fluorescence),
palynological (pollen grains), and paleogenomic (on plant and mammal sedimentary ancient DNA or sedaDNA)
data, which we based two 7.3 kyr-long sedimentary sequences recovered from the Kerall´e valley (Plouescat, NW
Brittany).
First, sedimentary and vegetation trajectories allowed us to reconstruct the evolution of past depositional
environments in the inner part of the Kerall´e valley. The data show a progressive sedimentary infilling of the
former alluvial plain, subjected to marine incursion between 7.3 and 6 cal ka BP, up to a continental area, totally
disconnected from tidal influence since 5.4 cal ka BP. Second, the analysis of plant and mammal sedaDNA
provided a complementary tool to the pollen analysis, enhancing our ability to accurately detect past land-use
practices. In the upstream part of the Kerall´e system, anthropogenic influence remained scarce until the late
Neolithic and seemed to be restricted to the coast, as also indicated by archaeological remains. The first sig-
nificant human-driven changes in plant community composition likely date back to 4.5 cal ka BP (i.e., late
Neolithic), as also shown by the presence of domesticated mammal sedaDNA, both being in agreement with the
documented settlement by the first metal-working societies (Bell Beaker culture) in the Plouescat area. From then
until 4 cal ka BP (i.e., early Bronze Age), human pressure became more obvious, with the steadily increase in
anthropogenic pollen indicators, while sedaDNA of plants and mammals were affected by preservation biases. At
2.7 cal ka BP (i.e., Iron Age), a new anthropic threshold was characterized by maximal forest clearing leading to
significant human-induced soil erosion across the Kerall´e watershed. Since then, at the local scale of our study,
both vegetation dynamics and the presence of domesticated mammals observed by sedaDNA attest to the
development of pastoralism.
Origine | Publication financée par une institution |
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